Microsoft has put limits on the types of PCs that can be downgraded to Windows XP after June 30, Dell said Tuesday, a restriction that means it will offer home users the option only on machines designed for gamers.

Buyers of the three consumer models that can be downgraded to XP after Wednesday must pay an additional $20 fee, Dell also said. The charge is similar to the $20 to $50 added to the price of Dell's entry-level business computers.

"Microsoft says what kinds of systems can be downgraded," said Dell spokeswoman Anne Camden Tuesday. "Those are enterprise, small business and gaming systems."

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While Microsoft has set June 30 as the general end-of-availability for Windows XP, Dell will stop pre-installing most versions of the seven-year-old operating system Wednesday. It will still ship some PCs with Windows XP, however, by taking advantage of the downgrade rights built into Vista Business and Vista Ultimate. Downgrading lets Dell install Windows XP Professional in lieu of Vista, although the newer operating system is still shipped with the machine so that buyers can, if or when they choose, move from XP to Vista.

"Small businesses are slow to transition to Vista," said Camden, who noted that business users often rely on applications that don't run in the newer OS. "Gamers are in [a] similar situation. They often have significant investments in special peripherals and special games that won't work in Vista."

After Wednesday, Dell will only downgrade three machines in its upper-end XPS consumer line, Camden confirmed: the XPS 630 and XPS 720 H2C desktops and the XPS M1730 notebook. Previously, Dell had said it would not offer XP as an option after Wednesday on its mainstream Inspiron lines of desktops and laptops.

A spokeswoman for Microsoft denied the company is keeping any customers from downgrading. "No, Microsoft is not targeting any specific customers with downgrade rights," she said in an e-mail. "Anyone who buys Windows Vista Business or Windows Vista Ultimate is eligible for a downgrade." Nor is Microsoft telling computer manufacturers which type of PCs they can downgrade to XP, she added.

In comparison to its limited number of consumer PCs that can be downgraded from Vista to XP after Wednesday, Dell's small business selection includes all four lines: Vostro, Latitude, OptiPlex and Precision.

Dell, which said Tuesday that it would charge up to $50 to factory-install an XP downgrade on its Vostro-branded desktops and notebooks, added later in the day that it planned to waive those fees for a limited time. "I can't say much more than 'limited time,' Camden said Tuesday afternoon, "but I wouldn't expect it to last more than a couple of weeks."

Buyers of Dell's Latitude laptops, OptiPlex desktops and Precision desktops and laptops don't incur any additional fee for downgrading.

The downgrade charge waiver for Vostro customers begins tomorrow, said Camden, who added that there will not be a similar deal for buyers of the three XPS machines that include the downgrade option.